In North America, train movement on mainline tracks is typically governed by fixed wayside signals. An enhancement of this system is the coded cab signal system utilized by carriers in areas that have a high traffic density. The coded cab signal system displays the current track signal aspect to the crew in the cab of the leading locomotive by inductively receiving and decoding a coded electrical signal in the rail. Typically, there are four signal aspects displayed, each indicating the condition of the upcoming track, such as whether there is another train or a divergent route ahead. Each of the four signal aspects has an associated maximum speed at which the locomotive may safely travel into the upcoming section of track. From the highest allowable speed to lowest, the four aspects are generally named: "clear", "approach-medium", "approach", and "restricting." The coded signal traveling within the track is typically a modulated 100 Hz signal, although other frequencies are sometimes used. The 100 Hz carrier signal is switched on and off, or modulated, at various rates to represent each signal aspect. Typical nominal modulation rates measure in cycles per minute are: clear =180 (3.00 Hz); approach-medium =120 (2.00 Hz); approach =75 (1.25 Hz); and restricting =no code (0.0 Hz). The nominal on/off duty cycle for the modulations is typically 50% on, 50% off, i.e., a 50% duty cycle.
The current of the 100 Hz signal in the rail induces a similar 100 Hz signal on the locomotive's receiver coils which are located above each rail. This induced signal is amplified and decoded either through the use of tuned relays or digital signal analysis to provide output to the cab signal display in the locomotive cab. The duty cycle, carrier frequency, modulation frequency, and amplitude of the track code all have tolerance bands associated with their nominal values. Operation of the system outside of these tolerances can result in erroneous, albeit more restrictive, signal indications being displayed in the locomotive cab.
Malfunctioning or marginal track code generating equipment usually causes incorrect signal indication, typically resulting in a "restricting" indication in the locomotive cab when a more favorable aspect should be displayed. Any downward change in signal aspect sounds an alarm and requires that the engineer acknowledge the change by stepping on a pedal. If the acknowledging pedal is not depressed within a specified time period, the train's brakes will automatically be applied. Besides unnecessarily delaying the train, unplanned applications of the brakes can cause high dynamic forces which, under the right conditions, can lead to a derailment. Since malfunctioning or marginal track code generating equipment usually causes a downward change in cab signal indication when one isn't expected, the proper functioning of the wayside track code generating equipment is essential for safe and efficient train operation.
Currently, deficient track code is only detected when the train crew notices the cab signal display showing the wrong aspect. Often the display will "flip" to a more restricting aspect and then "flip" back to the proper one, i.e., the signal aspect drops and returns to a higher indication in a short time. When this occurs, the operator does not know if the defect resides with the wayside equipment or the locomotive-mounted equipment. Once a problem has been reported, signal maintenance personnel must conduct testing of the wayside equipment to determine if the problem exists in that equipment. This is done by simulating train movements by lining switches and clearing signals with no trains present and then progressively shunting the track circuits in the route with an AC ammeter. The ammeter indication is then interpreted to determine the cab signal current and code rate.
In some cases, specialized devices have been developed to automatically determine the code rate, but the test procedure is essentially the same. Tests are normally preformed at the time of initial installation or modification of the signal system and at specified, periodic intervals and are intended to show the system is functioning as intended.
The above described methods for ascertaining the cause of a "flip" have four fundamental limitations. First, the tests do not measure certain parameters, such as the carrier frequency. Second, the tests are essentially static and do not give a good picture of transient conditions such as when a train passes from one track circuit to another at a high rate of speed. Third, the tests are simulations and not actual train movements and, therefore, many flips are not detectable with this method. Fourth, the wayside equipment is not tested between the specified test periods unless trouble is reported from a train.